Welcome to Android Central! What do you mean that the OS signs off? Do you mean it reboots? Or the screen turns off, and when you wake the phone, you have to enter your password or unlock pattern again? Does the call actually drop out?

Sorry for the delay... had to create an account and wait for verify... Yes, screen signs off and I have to re-enter pswd to unlock again. Doesn't drop call but only way to reactive usage is to press on button quickly then enter pswd.

Go to Settings>Security>Automatically Lock. This determines how long the phone waits after it goes to sleep before it requires you to unlock the screen again. If it says Immediately or 5 seconds, change it to something longer, like 1 or 2 minutes. The location of this option in your settings might differ--I'm using my Nexus 5 as a reference.